Aside from Summons, I can't think of any spell that my casters typically use during combat that have a casting time of more than 1 round. And if you are going to cast summon after summon, then once the first round goes by, then you get one per round, so it might as well be 1 action. I am surprised that it doesn't make the spell use a higher level slot like Quicken Spell does (perhaps it should), but overall I see nothing game breaking about it.
(also a level 4 caster can't use Rapid Spell as it requires 12 ranks of Spellcraft.)
There are a few CRB spells that would profit a lot from requiring "just" a full-round action, instead of 1 round:
Dominate Animal / Monster / Person
Enlarge / Reduce Person
Enthrall
Lesser Geas
Hypnotism
Silence
Sleep / Deep Slumber
Summon Monster / Nature's Ally / Swarm
These spells are (arguably) stronger than average, and barely held in check by the required casting time. Add to this the problem (IMO) that rods are too cheap, and you likely get a dominant strategy of summoners / enchanters running around with multiple rapid spell rods, weakening inter-party balance further and giving their GM more of a headache.
I'd be more open if it would +2 spell level - action economy improvements should have a steep price. The Sacred Summons feat might violate this already, but it has its own restrictions and having a foul apple already doesn't mean you should add another one without second thought.
That's actually pretty cool, although with this one feat just about anyone could have access to one of the Summoner's class-defining abilities (cast Summon Monster as a standard), and then effectively do it better than a summoner because now they can cast every other full round spell as a standard too.
Tbh, I'd make this +2 or +3 spell level if you're going to use it. Or, re-skin this feat as being able to cast a Standard Action spell as a Move Action, and probably go +2 or +3 spell level on something like this as well.
Psionics has a version of this called Hustle Power, allowing you to Manifest a Power as a Move Action, and it's really powerful but not broken. It increases the Power Point cost by 4 (4pp = to +2 spell levels), but you also have to expend your Psi Focus. So maybe +3 spell levels would be appropriate for a Metamagic version, or, you could have it cost +2 spell levels but cost their Move Action during this round to cast it, as well as their Move Action in the following round. In Psionics, regaining your Psi Focus costs you a Move Action in the following round, so that's pretty fair I think.
Hustle Power [Metapsionic]
You can hasten the manifesting time of some powers.
Benefit: To use this feat, you must expend your psionic focus. You may manifest a power with a manifesting time of one standard action as a move action instead. Powers with manifesting times other than one standard action may not be affected by this feat. You can perform another action, even manifest another power, in the same round that you manifest a hustled power.
Using this feat increases the power point cost of the power by 4. The power’s total cost cannot exceed your manifester level.
Manifesting a hustled power still provokes attacks of opportunity.
Special: This feat cannot be used on powers that cannot be quickened.